The present disclosure is directed to a built-in, self-test technique for MicroElectroMechanical systems (MEMS) that is applicable to symmetrical microstructures or other structures in which signals of equal magnitude and opposite polarity can be produced.
MEMS are complex, heterogeneous systems consisting of devices whose operation is based on the interactions of multiple energy domains. Commercial manufacture of MEMS has increased the need for cost-effective test methods that screen defective devices from good ones. With MEMS becoming increasingly complex and finding use in life-critical applications such as air-bags, bio-sensors, and aerospace applications, there is a growing need for robust fault models and test methods.
Of the currently used MEMS process technologies, surface micromachining is a popular one due to its well-developed infrastructure for depositing, patterning and etching of thin films for silicon integrated circuits. Surface micromachining enables the fabrication of high-quality MEMS devices because it is based on thin-film technology that combines control and flexibility in the fabrication process.
A MEMS test may include the process of identifying good devices in a batch of fabricated devices. The normal assumption is that the design is correct so the test process is one of verifying that the fabricated device is equivalent to the design. However, a device that passes a traditional, specification-based test may fail later during in-field operation. For example, a mechanical beam of an accelerometer may become stuck to the die surface due to a phenomenon known as stiction. A stuck beam may mimic behavior similar to a device affected by an expected level of under-etch. Under-etch refers to a release step where sacrificial material is removed to release or free the microstructure. The release typically requires an etching step, and in the case where an under-etch occurs, insufficient sacrificial material may be removed. By adjusting the electronics, an accelerometer suffering from stiction can be easily calibrated to meet its operational specification. The danger, however, is that an accelerometer with a stuck beam may release in the field (i.e. defect healing) causing the accelerometer to go out of calibration, which can then possibly lead to failure. Detection or prevention of field failures can be accomplished through built-in self test (BIST).
As MEMS become more complicated and find a wider range of applications, the need for on-chip self-test features will grow. BIST for the testing of MEMS is yet to be common practice. However, progress in this area has been recently made. The work in De Bruyker et al., “A Combined Piezoresistive/Capacitive Pressure Sensor with Self-test Function based on Thermal Actuation,” Proc. Solid State Sensors and Actuators, Vol. 2, pp. 1461–1464 (1997) describes the self-test of a pressure sensor. In their approach, thermal actuation of the sensor's diaphragm is performed by driving current through a resistive heater. The heat generated increases the temperature of the air in the sensor's cavity creating a pressure that displaces the diaphragm. Using a similar technique, the authors Charlot et al., of “Electrically Induced Stimuli for MEMS Self-Test,” Proc. VLSI Test Symposium, pp. 210–215 (April–May 2001) use resistive heaters to increase the temperature of a MEMS infrared-imager array. Many commercial accelerometers use a self-test technique similar to the one described in Allen et al., “Self-Testable Accelerometer Systems,” Proc. Micro Electro Mechanical Systems, pp. 113–115 (1989). In that approach, dedicated mechanical beams are used to generate an electrostatic force that mimics an external acceleration. It is useful for determining if the accelerometer's mechanical microstructure is free to move. This technique cannot be used until the electrostatic force is calibrated after testing has been performed to determine if the part has been manufactured correctly. Finally, an idea for accelerometer self-test that exploits design symmetry is suggested in Rosing et al., “Fault Simulation and Modeling of Microelectromechanical Systems,” Computing and Control Engineering Journal, Vol. 11, Issue 5, pp. 242–250 (October 2000). They propose to actuate the accelerometer one side at a time and then compare the two outputs obtained to detect any anomaly.
Commercially-manufactured devices such as accelerometers are usually affected by multiple failure sources. Failure sources for MEMS include, but are not limited to, foreign particles, etch variations, and stiction, each of which can lead to a variety of defects. For example, it is known that particles can lead to defects that include broken and bridged structures with corresponding behaviors that range between benign and catastrophic. Many of these failure sources exhibit very similar misbehaviors and are difficult to distinguish from each other.
Currently, self-test of commercial accelerometers is limited. BIST techniques used in industry (See Allen et al., “Self-Testable Accelerometer Systems,” Proc. Micro Electro Mechanical Systems, pp. 113–115 (1989)) are focused on input stimulus generation. In accelerometers produced by Analog Devices, Motorola and others, the accelerometer's shuttle is moved to its maximum position using actuation fingers so that the full-scale sense output is generated. The inability to generate a full-scale output, within some tolerance limits, means the accelerometer has failed self test. Using this form of BIST for testing is difficult because the amount of actuation voltage needed can be determined only after the part has been tested and calibrated. It is also ineffective for distinguishing misbehavior stemming from different sources. For example, a BIST output that is larger (smaller) than expected can be either caused by over-etch (under-etch) or broken (stuck) beams. Hence, its ability to identify hard-to-detect defects (e.g., asymmetry due to local defects) is limited.
In the BIST technique proposed in Rosing et al., supra, the accelerometer's shuttle is moved twice, once using the right actuation fingers and again using the left actuation fingers. Failure results when the two resulting sense outputs do not match, presumably, within some tolerance level. Unlike the techniques currently used in industry, this method does not necessarily require calibration before it can be used. However, its ability to identify hard-to-detect defects is limited because all test observations are made from the normal sense output. Moreover, it is difficult to implement on chip because sample-and-hold circuitry is required to store the first measurement.